1. Field of the Invention
The present invention generally relates to electron beam (e-beam) deflection and exposure systems and, more particularly, to electron beam lithography for the high accuracy exposure and patterning of resist masks during the manufacture of semiconductor devices.
2. Description of the Prior Art
In order to achieve high accuracy in the deflection of an electron beam, multiple stage deflection systems have become well-known in the art in a range of devices including high resolution displays. In particular, electron beam lithography has been used in the exposure of sensitized resist materials for the formation of masks to cause selective etching or material deposition in the processing of semiconductor wafers and chips for a number of years. Due to the small size and high accuracy with which selected areas must be exposed and the speed requirements of manufacturing processes for practical throughput rates, the demands on electron-optical deflection systems have been particularly stringent. In order to maintain the accuracy required in the electron-optical deflection system, it has often been necessary to employ deflection arrangements which are not capable of covering the entirety of the desired writing field. In such a case, the writing field is normally divided into sub-fields which are then separately written in succession, altering the position of the target or the coarse positioning of the electron beam from sub-field to sub-field. Such a division of the writing area can be done in three or more levels if necessary to achieve the desired accuracy and speed of electron beam exposure.
The relationship of the writing area and sub-fields is illustrated in FIG. 1, with respect to which some terminology which will be used in this application will be defined. The entire field or writing area 10 on which an image is to be formed is indicated by outline 12. Writing area 10 is divided into a matrix of sub-fields, a nominal sub-field is indicated at 14. The location numbers of sub-fields is conventionally numbered in the x direction as 0 through n and in the y direction as 0 through m. The number of sub-fields is arbitrary, as is the number of rows and columns of the matrix into which writing area 10 is divided. The only constraint on the division of writing area 10 is that the dimensions of each sub-field must be less than the maximum deflections available from a given (e.g. most minor) level of deflection available in the electron beam deflection system. The difference between the dimensions of the maximum deflection of that level of the deflection system and the chosen sub-field dimensions yields a margin or extension 16 for the sub-field. Since the adjacent sub-fields, by definition, are contiguous, the sub-field extensions overlap adjacent sub-fields. The dimension of the mutual sub-field extension overlaps of adjacent sub-fields 18 is commonly referred to as an overlap zone. For pairs of rows or columns of sub-fields, the aggregate area of overlap is referred to as a y-overlap zone or x-overlap zone, respectively, The term direction overlap zone is generic to x- and y-overlap zones.
To form patterns in each of the sub-fields, it is the common practice to form the desired e-beam exposure pattern of rectangles, each of which may correspond to one or more spot rectangles or spots, as shown for an arbitrary pattern in FIGS. 2a and 2b, which will also be used to define some of the terminology used in this specification. FIG. 2a shows an arbitrary shape to be exposed by the e-beam exposure system, as indicated by outline 21. Known arrangements for fitting rectangles into a desired shape are also able to outline the shape with rectangles such as rectangles I-IV which are then referred to as "exterior". This operation is called "sleeving". Exterior rectangles also include rectangles such as rectangle V which is not sleeved since edges thereof are less than a split dimension apart which, in turn, is usually less than a maxspot dimension which is the maximum transverse dimension in either coordinate direction which can be produced by the e-beam exposure tool. The other spot rectangles tiling the shape but not located at a pattern edge such as rectangle VI are referred to as "interior". The distinction between exterior and interior rectangles is useful for purposes of producing an exposure since accuracy of exposure "dose" (e.g. the product of time and intensity of the exposure) is less critical for interior spot rectangles than for exterior spot rectangles.
This difference in sensitivity to dose accuracy exists because sensitized films such as resists will tend to exhibit blooming if overexposure is especially severe. This effect can also be observed with photographic films and phosphors of display tubes (and can also be aggravated by secondary emission effects in such devices). In patterning of resists as masks, the film should reach a saturation level of exposure to develop full contrast (e.g. to avoid grey level areas in the completed exposure). However, if the exposure dose is significantly greater than that required to saturate the material, adjacent areas may behave as if exposed as well, resulting in blooming and a loss of dimensional accuracy and edge definition in the resulting image if the exposed area is an exterior rectangle. Any blooming of interior rectangle, however, will only be masked by an exterior rectangle if the width of exterior rectangles is greater than the width of the "bloom". Typically, the blooming of interior rectangles can be reasonably limited and the sleeve dimension shown in FIG. 2 is chosen to fully mask the effect.
As noted above, the spot rectangles each have a maximum dimension in a coordinate direction which is limited by the beam shaping of the electron optical arrangement. This maximum dimension is referred to as the maxspot dimension or, simply, maxspot. The term "maxspot" is also used to refer to any spot rectangle which has one of two coordinate dimensions equal to the maxspot dimension. In FIG. 2b, for example, maxspots are marked with an F to indicate that they are full maxspot size in at least one coordinate direction. Compare, for example spot rectangles M1 and M2 which are of differing heights but are both of a maxspot dimension in the x-direction.
If required by the desired pattern, spot rectangles having one or both orthogonal dimensions less than the maxspot dimension can be formed. The maximum dimension in either or both coordinate directions may be reduced as required by the pattern to be exposed. A spot rectangle in which neither dimension is as great as the maxspot dimension is referred to as a remainder spot as illustrated at R1 and R2 of FIG. 2b.
The rectangles, each including one or more spot rectangles, which form the pattern are non-overlapping and tiled into the shape during sleeving from left to right and top to bottom, as illustrated in FIG. 2a. It is to be understood that the scale of FIGS. 2a and 2b is much enlarged from that of FIG. 1 and the non-overlapping rectangles of FIG. 2, identified by Roman numerals therein are not to be confused with the rectangular sub-field areas of FIG. 1. Similarly, spot rectangles are not to be confused with the rectangles (e.g. I-VI) defining the pattern.
It is also to be understood that, in practice, the rectangles forming spots and hereinafter referred to as spots or spot rectangles, as produced by the e-beam device, will often have an aspect ratio generally in the range of 2:1 to 5:1 since the sleeve dimension is normally set in accordance with the best focus of the e-beam exposure apparatus. (The ends of the spots in the direction of maxspot length typically abut another rectangle and focus is less critical.) For example, the largest (e.g. maxspot) spot rectangles illustrated in FIG. 5 have an aspect ratio of 3:1. This relatively high aspect ratio in FIG. 5 is a result of the pattern to be formed and the split distance, referred to above which is, by definition, the maxspot size or less. The split distance is a predetermined maximum width of a pattern portion in which slleving is not permitted. Therefore the entire width of the pattern is exposed by a single spot having a width which is less than the maxspot size. If the pattern permits, sleeving and tiling of the pattern could consist primarily of spot rectangles which are of maxspot size in both coordinate directions. However, the sleeve dimension is often chosen to be less than the split dimension to reduce the number of exterior spot rectangles which must be produced when the pattern has many fine details.
Under normal circumstances, exposure doses can be fairly well-regulated within a sub-field. However, when the entirety of a pattern cannot be formed within a single sub-field or a sub-field plus its extension, registration problems from many possible causes may result in errors of occlusion between the portions of the pattern exposed from respective sub-fields. Therefore, it is common in the art to carry the pattern to be written from each of the respective sub-fields into the respective sub-field extensions at a reduced exposure dose. The overlap of rectangles produced according to the respective sub-fields will then reduce the likelihood of discontinuities in the exposure pattern and more reliably form the desired pattern and corresponding product. The process of creating this overlap at reduced exposure is referred to as grey-splicing, grey-splice, greying or, simply grey. (An alternative spelling, "gray" is also used in such terms and refers to the same process.) This process is quite critical since the exposure dose contribution from each sub-field must be kept within close limits to avoid blooming from the multiple exposure while keeping total exposure levels close to saturation even though correct registration cannot be assumed. In other words, if the occlusion errors cause a non-greyed area to overlap with a greyed or non-greyed area of another sub-field, substantially excess exposure will result. Conversely, if the sub-field areas are separated, areas in the gap between the sub-fields may receive only a greyed exposure dose corresponding to a rectangle of only a single sub-field or, in some circumstances, no exposure at all.
More specifically, known arrangements for controlling exposures in regard to a plurality of sub-fields require that each interior and exterior rectangle be assigned to a specific sub-field. If a rectangle cannot all be written from a single sub-field plus its sub-field extension, the rectangle must be cut into a series of smaller rectangles. This can be visualized with reference to FIG. 3 which is an enlarged portion of FIG. 2 in the vicinity of dashed line 20 which represents a location of a boundary between sub-fields. The limit of the sub-field extension of the sub-field to the left in FIG. 3 is indicated by chain line 32. The limit of the sub-field extension of the sub-field to the right in FIG. 3 is indicated by chain line 30. The boundaries of rectangles I, III, V and VI are indicated by heavy lines and the rectangles are further divided into maxspot and remainder rectangles as in FIG. 2b.
It can be seen that the right edge of rectangle I lies outside the sub-field in which the majority of the rectangle lies. However it can be written entirely from the left sub-field. Alternatively, rectangle I could be cut at any maxspot boundary 21, 23, 25 and the right end of rectangle I could then be written from either or both sub-fields. Additionally the leftward extent to rectangle I may be sufficiently great as to cross further sub-field extension boundaries, requiring further cutting.
Rectangle III lies entirely in the sub-field to the right of sub-field boundary 20. However, since it also lies to the left of sub-field extension boundary 32, the entirety of rectangle III could also be written from either or both sub-fields. Rectangle V lies entirely in the same sub-field as rectangle III but crosses a sub-field extension boundary. Therefore, rectangle V can only be written from the sub-field to the right of FIG. 3.
It should be noted that a single division of a rectangle does nothing toward compensating for errors of occlusion between sub-fields. Therefore, to provide such compensation, rectangle I would be typically divided into a plurality of rectangles, each of which will be written at full dose or greyed dose levels from each of the sub-fields. It should also be noted, as briefly referred to above, that the grey-splicing process will be carried out only on exterior rectangles to reduce processing overhead although division of interior rectangles will be done in a similar manner. In any overlap of rectangles involving at least one interior rectangle, the interior rectangle can be done at full dose since any resultant blooming will be contained within the shape outline sleeved by the exterior rectangles.
In order to understand in detail how rectangles are divided and assigned to particular sub-fields for control of e-beam exposure including greying, a known method of grey-splicing will now be discussed. One such methodology comprises the following steps:
1. Assign any rectangle that can be written from one sub-field to the sub-field (e.g. the sub-field and its extension) where the majority of the rectangle lies. If a rectangle is too large to be written from one sub-field, divide it into pieces, nominally along sub-field boundaries, which can each be written from a single sub-field and assign each piece to the sub-field it resides in. PA1 2. Sort rectangles by x- and y-location and place the rectangles into a queue. PA1 3. For each rectangle in the queue, find all abutting (e.g. contiguous) rectangles. Then, for each rectangle abutting another neighbor rectangle, PA1 4. Repeat for each neighbor of the queue rectangle. PA1 4. Repeat for each rectangle in the queue.
If the neighbor rectangle is assigned to the same sub-field as the queue rectangle, no action is taken, PA2 If the neighbor rectangle is assigned a different sub-field than the queue rectangle, and the borders align exactly, then adjust border of the rectangles such that the left (or top) rectangle is a multiple of the maxspot and perform greying on the overlapping area. This is illustrated in FIG. 4a. PA2 If the neighboring rectangle and the queue rectangles align such that the abutting edge of one rectangle spans (e.g. contains) the other, overlap the smaller edge into the larger edge by a user specified distance at full dose. In this case, blooming of the overlapping shorter edge will be substantially contained within the rectangle having the longer edge. This is illustrated in FIG. 4b. PA2 If the neighbor rectangle and the queue rectangle align such that neither edge fully contains the other, no action is taken. This is illustrated in FIG. 4c. PA2 END for queue rectangle.
It is to be understood that the locations of abutting edges in the foregoing methodology are described in terms of ideal, intended locations of rectangles in the vicinity of ideally contiguous edges of adjacent sub-fields. Therefore, no gap between adjacent rectangles exists in this ideal description and the overlap provided in some cases is intended to compensate for errors of occlusion between sub-fields when the exposure is actually carried out.
Although the above methodology has produced good results in many situations, two problems have occurred which are attributable to this methodology. First, in some cases where no action is taken and others, if the occlusion error is large, separation of rectangles may, nevertheless, occur. Second, when overlap is performed at full dose, blooming will be produced in the area of the overlap. In some cases, overlap at full dose may be tolerable as will be discussed below with reference to FIG. 4b. However, when overlap occurs due to occlusion errors, blooming will be produced which causes edge resolution to be lost and may result in a defective pattern.
The result of various errors of occlusion are illustrated in FIGS. 4a'-4c' and 4a"-4c". For purpose of discussion of these Figures, it is irrelevant which rectangle is the queue rectangle and which is the neighbor rectangle, the dimensions of which are labelled Q and N.
Specifically, if the occlusion error causes the sub-field to overlap, the conditions of FIG. 4a would cause excess exposure in the cross-hatched area of FIG. 4a' and blooming 41 would result in the location shown. Similarly in the circumstances of FIG. 4b where the overlap by a user selectable distance yielded little or no blooming in areas 43, the increased overlap of sub-fields would cause blooming 41 to occur. (Note that the overlapped region of the narrow rectangle is exposed at full dose from both sub-fields) Conversely, an occlusion error causing separation of sub-fields, under the circumstances of FIG. 4a would yield an acceptable pattern unless the occlusion error was very large as shown in FIG. 4a" (separation in the finished product may be greater than the gap shown since the abutting edge areas of the rectangles receive a greyed exposure dose. However, under the circumstance of FIG. 4b, where the distance d is user selectable to minimize blooming at proper occlusion, separation could more easily result as shown in FIG. 4b".
Under the circumstances of FIG. 4c, where no greying or alteration of rectangle dimensions is done and overlap of sub-fields causes an overlap of rectangles, blooming 41 occurs, as indicated. If occlusion error causes separation of sub-fields, separation of rectangles occurs.
Therefore, it is seen that the prior art greying methodology yields acceptable results only in one of the six possible occlusion error/greying scenarios, assuming the occlusion errors are greater than d but less than the maxspot dimension. In fact, this methodology is viable only because the blooming of FIG. 4b' will seldom be significant to the pattern since it occurs on inside corners of the pattern (conversely, d can be made somewhat larger and some blooming tolerated in order to reduce the frequency of separations), and because the juxtaposition of rectangles shown in FIG. 4c is relatively rare in actual designs.
In summary, whenever the prior art provides some compensation for occlusion errors between sub-fields in order to reduce the frequency of occurrence of patterning separations, a risk of blooming of portions of the pattern is engendered.